The present invention relates generally to a laser diode array. More specifically, the present invention relates to a laser diode array having integral structures that facilitate the measurement of optical power generated by the laser diode array.
Laser diodes degrade with time and use. Therefore, the current supplied to the laser diode to produce a predetermined optical power at the beginning of life of the laser diode will be different from the current needed to produce the same predetermined optical power at the end of the effective life, i.e., just prior to failure, of that laser diode. It will be appreciated that this fact gives rise to the need to recalibrate the Power vs. Current curve and subsequently control these parameters over the operating life of the laser diode array. This laser diode aging problem is exacerbated when the device being controlled is not a single laser diode but a laser diode array constructed from many laser diode bars, each laser diode bar including a plurality of distinct laser diodes. If one bar fails, then the array continues to operate but at a reduced output level, which could adversely affect the quality of the product being manufactured with the laser.
Moreover, it is often desirable to be able to directly monitor various performance parameters of a laser diode array or system. These performance parameters can include, but are not limited to:
(i) power output; PA1 (ii) optical activity or output; PA1 (iii) wavelength detection and tuning indications; PA1 (iv) laser operation data of the pumped laser using light traveling back from the laser medium in the cavity to the diode pump source; PA1 (v) pulse output characteristics of pulse-driven diode pump sources; and PA1 (vi) feedback from damaged optical fiber links for detection and diagnostic purposes. PA1 (i) a beam sampler significantly increases the size of the package; PA1 (ii) a beam sampler placed in the path of the emitted light attenuates a portion of the available light, which might otherwise be delivered; and PA1 (iii) a beam sampler increases the cost of the system.
Coupling optics have been used for coupling output emissions of a laser diode pump source to an optical fiber. An example of such coupling optics is disclosed in U.S. Pat. No. 5,127,068 (hereinafter the '068 patent), which is incorporated herein by reference. In the '068 patent, radiation emitted from a laser diode bar, constructed from a plurality of emitters, is focused into a corresponding number of multi-mode optical fibers by means of coupling optics placed between the emitting facets of respective emitters and the ends of the multi-mode optical fibers. The coupling optics disclosed in the '068 patent is simply a piece of the same multi-mode optical fiber, a strand of which extends along the length of a diode bar pump source. It will be appreciated that the coupling optics must be carefully positioned with respect to the emitter facets of the laser diode bar in order to properly collimate the emitter outputs. This is accomplished by aligning and securing the coupling optics in place with a suitable epoxy material.
Diagnosis of the performance characteristics for the system discussed immediately above are generally achieved by placing a beam sampling device, e.g., a beam splitter, in the path of the emitted light between any two of the diode pump source, the coupling optics, and the optical fiber. This diagnostic technique presents at least three problems:
Moreover, even if the problem of limited space in the laser diode system were ignored, building a feedback subsystem for such a system would require extra mechanical components for stability. Such stabilizing elements add complexity to the feedback system as well as increase the weight and cost of the overall system.
In an attempt to overcome the problems mentioned above, U.S. Pat. No. 5,504,762 (hereinafter the '762 patent), which is also incorporated herein by reference, proposed employing the optical fiber constituting the coupling optics to conduct stray radiation, which is not included in the beam that is generated in the emission path by small imperfections in the surface of the lens, to an alternate location. The optical fiber, having a first end and a second end, is oriented with respect to the coupling optics such that radiation from the emitter region is optically coupled into the optical fiber. The second end of the optical fiber is coupled to the laser cavity. A detector is positioned in a spaced, adjacent relationship to the coupling optics, i.e., in an emission path of the stray radiation. The detector detects at least a portion of the stray radiation, and produces a detected output in response to the stray radiation.
More specifically, FIG. 1 discloses an arrangement for coupling the radiation emitted from laser diode source 10, which has a single emitter region, or a bar, having a plurality of emitters 20, 22 and 24, into multi-mode optical fibers 26, 28 and 30, which fibers are part of an optical fiber bundle 32. The arrangement includes a coupling optics 18 disposed between the emitter facets of emitters 20, 22 and 24, and the ends of multi-mode optical fibers 26, 28 and 30. It will be noted that it is not necessary to couple a certain number of laser diode emitters to an equal number of fibers, fiber bundles, or other optical transfer elements. It is possible to overlay power from more than one emitter to a smaller number of fibers, fiber bundles, or optical transfer elements. It will also be noted that optical transfer devices can extend beyond fibers and can be substantially any optical device or media, either reflective, refractive or open media which is placed at the output of laser diode source 10 to allow the output to be effectively coupled to a desired application. Air is only one of many transfer elements. Other suitable transfer elements include fluids, liquids, and solids.
It should be mentioned that a spacing of approximately 60 microns from the near edge of coupling optics 18 to the diode facet is satisfactory for a 250-micron diameter fiber that has an index of refraction of 1.5. The optical spacing of the optical fiber end from coupling optics 18 should be as small as possible; a spacing of about 20 microns is acceptable.
It will be noted that the diameter of coupling optics 18 is chosen to be roughly equal to the diameter of the optical fiber to be coupled. The diameter of coupling optics 18 may be less than the diameter of the optical fiber to be coupled without loss in coupling efficiency. However, if such small coupling optics 18 diameters are used, the alignment of coupling optics 18 is more difficult. Coupling optics 18 is placed with respect to the output facets of laser diode source 10 in order to properly collimate them. This may be accomplished by carefully aligning coupling optics 18 and securing it in place with a suitable epoxy.
The NA of the optical fiber to be coupled is roughly equal to the low NA direction of diode source 10, typically 0.1 to 0.15. This combination of coupling optics 18 and optical fiber results in a percentage greater than 80% coupling of the laser diode emitted energy into the multi-mode optical fiber. Coupling optics 18 and the butt-coupled end of the fiber have an anti-reflection coating to reduce reflections from these surfaces.
It will be noted that coupling optics 18 is cylindrical in cross-section. Those of ordinary skill in the art will recognize, however, that other cross-sectional shapes, including but not limited to, elliptical and hyperbolic, which can be useful for correction of particular spherical aberrations as known in the art, i.e., Kingslake, Lens Design Fundamentals, Academic Press, 1978.
As illustrated in FIG. 1, the optical fibers 26, 28 and 30 are rectangular in cross-section in order to reduce the total amount of glass in the fiber. The width of the rectangular fiber is chosen to be slightly larger than the diode emitting area The height is as small as possible, typically around 30 to 50 microns. It should be noted that although heights smaller than 30 to 50 microns may be used, such heights make alignment of the system more difficult. The diameter of coupling optics 18 is equal to the height of the rectangular fiber, e.g., 30 to 50 microns in the example given. The resulting fiber output is brighter since the total emission area is smaller.
Each emitter region 20, 22, and 24 collectively produces an emitter region emission path of radiation 34 that is optically coupled to optical fibers 26, 28, and 30. Stray radiation is emitted from coupling optics along a stray radiation emission path 36. In FIG. 1, stray radiation emission path 36 is generally perpendicular to emitter region emission path 34. The light that is transmitted along stray radiation emission path 36 is not used to pump the laser medium.
Referring now to FIG. 2, a detector 38 is positioned in a spaced, adjacent relationship to the coupling optics 18, along stray radiation emission path 36. When coupling optics 18 is a fiber, detector 38 is positioned at one end, or both ends, of the fiber in an emission path of stray radiation, which is generated by coupling optics 18. Detector 38 is a semiconductor photodiode, such as a semiconductor photodiode manufactured of materials including but not limited to silicon, germanium, alloys of gallium arsenide, and the like. Positioned between detector 38 and coupling optics 18 is an optical filter 40, which filter eliminates visible light but passes infrared light. Detector 38 is attached to filter 40 to prevent variations due to alignment. Detector 38 detects at least a portion of the stray radiation from coupling optics 18, and produces a detected output in response to the stray radiation.
Coupled to detector 38 is a signal processor 42, e.g., a digital voltmeter, a sensitive amplifier with fixed or adjustable gain which can be constructed from inverting, noninverting amplifiers, comparators or difference amplifier components. Signal processor 42 receives the detected output from detector 38 and generates a processed output that is representative of a detected characteristic of the laser system associated with diode pump source 10. A controller 44 couples the processed output from signal processor 42 to a diode power supply 46. A computer 48 can be included and associated with controller 44. Computer 48 includes a CPU coupled through a system bus. This system bus can include a keyboard, disk drive, or other non-volatile memory systems, display, and other peripherals, as known in the art. Also coupled to the bus are a program memory and a data memory.
From the discussion above, it will be appreciated that the feedback system for a laser diode bar cannot be readily adapted to a laser diode array constructed from a plurality of laser diode bars. For example, in order to sample the output of several laser diode bars 10, each of the laser diode bars 10 would have associated therewith a separate coupling optics 18. In addition, it will be noted that the each filament of the coupling optics 18 has an associated mechanical coupling device (not shown) to connect to the detector 38 and that additional structural elements must be included in the overall system to provide additional rigidity to the ends of the coupling optics 18 proximate to the detector 38 in the laser diode system depicted in FIG. 2.
It would be highly desirable to provide a feedback system that is much simpler than those currently proposed. It would also be highly desirable to provide a sampling mechanism that is more robust than those currently proposed. There is a need for a feedback system that would not occupy the limited space between any two of the diode laser source, the coupling optics, a fiber bundle, a laser gain medium, and the focussing optics. There is also a need for a sampling mechanism which does not use light intended for use in pumping the laser medium as an input. In particular, there is an attendant need for a sampling mechanism which permits sampling of the output of the entire laser diode array without substantially impacting the footprint of the array, which would allow the arrays adjacent to one another in a multiple array system to be mounted with minimal separation. It would also be highly desirable to be able to selectively detect the optical power coming from the whole laser diode array or the optical power generated by any one of the individual laser diode bars in a laser diode array.